The present invention relates to switching systems and more particularly to an electronic switching system for digital information. Such a system may be embodied in an electronic private automatic branch exchange (EPABX) of a telephone system, for example.
A telephone private branch exchange (PBX) is usually located on the premises of a telephone system customer to provide for switching calls between any two extensions served by the PBX or between any extension and the outside telephone system via a trunk to the telephone central office.
Historically, the small business PBX telephone requirements were served by a manually operated cord-and-jack switchboard. Such equipment was bulky and unsightly and required the undivided attention of the operator. The cord-type manual PBX has jacks, each with an associated lamp and designation, for each extension and central office trunk. Cord circuits terminated in pairs of flexible cords tipped with plugs are used to connect the jacks to establish the desired connections. The attendant has a telephone and dial circuit which can be connected to any cord circuit by operation of appropriate keys.
An improvement on the basic cord-and-jack manual PBX is the cordless manual PBX in which the operator uses keys to operate relays, which perform the actual switching function. In the cordless manual PBX, the switching equipment is located remote from the operator console, thus providing a more pleasing appearance.
A further improvement over the manual PBX is the automatic PBX, or referred to as a "PABX" or dial PBX, in which each extension is equipped with a dial so that the party may set up his own call without intervention of the operator. Early types of the PABX employed electromechanical switching similar to that used in central offices: step-by-step switching and crossbar switching. Such switching systems required substantial space, soundproofing, and special power plants.
The third-generation in PBX design is the electronic PABX (EPABX) in which the switching is processed electronically. Prior art electronic PABX systems fall generally into several categories: space-division switching, time-division switching, and hybrid space-division/time division switching. Such systems may further be categorized as analog and digital according to the condition of the information as it passes through the switching system. A typical characteristic of the electronic PABX is common control circuitry of some type.
In the space-division type of EPABX a switching network of cross points is controlled by a central control logic. The crosspoints are typically reed switches or solid-state devices. A close analog to electromechanical crossbar switching is apparent. A two way communication path in real time is provided in the conventional way.
In a variation of the space-division EPABX, one prior art system employs time-division multiplexing and demultiplexing at the inputs and outputs of a space-division network to provide a hybrid space-division/time-division system.
Prior art time-division EPABX systems provide a time-division multiplex "highway" or bus common to all lines and trunks. The highway passes analog information on a time-division basis using sampling techniques. A central control establishes time slots in which particular lines or trunks are gated to provide connections via the common bus. One such system having analog sampling is described in "Economical Electronic Switching for the Small Business User" by Tony Stansby in Telesis, Vol. 2, No. 3, Winter 1971.
Modification of the analog time-division multiplex system of the aforementioned Telesis article are disclosed in U.S. Pat. 3,652,800 issued Mar. 28, 1972 to Gerald F. Dooley and U.S. Pat. No. 3,806,654 issued Apr. 23, 1974 to Gerald F. Dooley et al. The basic analog time-division switching matrix is made to accommodate digital signals by controlling the sample rate and bandwidth of the digital signals. As in the Telesis article, the switching arrangement or matrix is external to the central control means. That is, the central control is a separate adjunct to the information transmission path.
Another type of time-division multiplexing which is used in communications should be noted. In telephone carrier systems and microwave links, dedicated ports are connected in a time-shared manner. Port A.sub.n always talks to port B.sub.n and so on. Such as approach is obviously not applicable to a telephone switching system in which flexibility is essential.
It will be apparent that electronic switching systems have departed conceptually in large degree from conventional electromechanical switching systems. Space-division systems while providing certain advantages retain many of the disadvantages of the earlier electromechanical cross point systems such as the need to test multiple paths and the occurrence of path blocking long before the network is fully loaded. A large space division system is also subject to crosstalk depending on the quality of the switch and time complexity of the possible interconnections.
Time-division switching can overcome such shortcomings inherent in cross point switching. For example, by assigning a time slot only when service is acquired, the system can be "filled to the top." That is, blocking does not occur until every time slot is in use.
Prior art time-division switching systems have heretofore been limited in the number of possible lines. For example, the pulse amplitude modulating (PAM) bus of the system of the Telesis article is inherently limited to a relatively small number of direct interconnections. Further secondary switching of this type, while providing more interconnections, introduces further signal distortion and increases system complexity. Moreover, such as system becomes further restricted when multiple slots are required for handling digital signals. Analog cross-talk between time slots remains a problem. The number of time slots are limited by the need to transmit sufficient analog information while still not showing interaction between different samples.
The advantages of time-division switching can best be utilized in a system designed from the "ground-up" to handle digital information. Such a system provides the flexibility of handling not only analog-to-digitally converted voice information but also direct digital information as, for example, between a time shared computer and a remote terminal. Also, such a digital system simplifies connection with the standard U.S. telephone T1 trunk which is in digital format.